Ding ding! Self etching is for bare surfaces - metal, fiberglass, some plastics. It's there to provide a hold for the next layer of primer to adhere to rather than the bare material. You really don't want to apply your paint directly to the self-etching primer. It can handle a little rust, like after a wire wheel or whatever but you wouldn't want to hose it over and expect it to seal in a ton. For that you should use rust inhibitor first. You also don't want to bondo on top of it.
The pros with spray booths and guns will tell you to use epoxy primer instead, which I would agree with. But when you are bomb canning, etch is fine as long as you top coat it with a sandable primer.
He's using a bomb can red oxide, not the brush on stuff like they use on bridges. It provides some rust protection, but is really just a sandable primer with some inhibitors. You absolutely still need to apply rust inhibitor like Rust-Mort and/or hit it with a wire wheel. Then the primer. Otherwise, the ghost of rust past will haunt you.
I like to use a filler primer when I have some rust pits. It fills them in nicely once sanded. Also glazing putting will be your new best friend when long boarding. It's much more forgiving than bondo and fills bondo pinholes nicely.
Also, primer is not UV resistant or a sealer. It will get chalky in the sun and let water through eventually.
